Evaluating the measurement signal of vacuum leak detectors

Abstract

A method for evaluating a measurement signal of a vacuum leak detector is provided. [Solution] [0010] TIFF2024526892000032.tif81154

Description

[Technical field]

[0001] The present invention relates to a method and a device for evaluating the measurement signal of a vacuum leak detector. [Background technology]

[0002] A vacuum leak detector is used to detect leaks in a test object. For this purpose, the vacuum leak detector includes a test chamber for placing the test object, a vacuum pump connected to the test chamber for evacuating the test chamber, and a gas detector connected to the test chamber and configured to analyze gas drawn from the test chamber. Specifically, the gas detector is configured to detect the test gas present inside the test object, which enters the test chamber through a leak in the test object and is supplied from the test chamber to the gas detector.

[0003] The suction capacity of a vacuum leak detector for the test gas used (most often helium) is determined by the vacuum pump used and the cross-sectional area and length of the connection to the test chamber (i.e. the volume of the gas channel between the test chamber and the gas detector). This effective suction capacity of the test gas determines the system time constant of the vacuum leak detector for the volume of the test object. The system time constant is the time of the system-related rise of the measurement signal due to the gas sensor reacting to a gas component or the time of the system-related fall of the measurement signal due to the absence of the gas component at the gas sensor. The gas component at the gas sensor is typically the component of the test gas that escapes through the leak and is to be detected for leak detection. Generally speaking, the system time constant is considered to be the time that elapses until the measurement signal falls to a rate of 1 / e or the measurement signal rises to a rate of (1-1 / e) when the test gas is sprayed or opened at the leak or when the spraying of the leak is stopped or closed. For simplicity, the vacuum time constant of the vacuum leak detector (the volume of the object under test or test chamber divided by the effective suction capacity of the vacuum leak detector) is sometimes considered to be the system time constant. Summary of the Invention [Problem to be solved by the invention]

[0004] Suction capacity can be improved by increasing the size of the pump or increasing its volume or cross-sectional area, which improves capacity / performance and response time. However, pump volume, cross-sectional area and dimensions cannot be increased at will. Moreover, increasing the volume reduces the amplitude of the leak signal, making leak detection more difficult.

[0005] It is an object of the present invention to provide a method and apparatus which allows for a shorter response time and improved signal evaluation. [Means for solving the problem]

[0006] A method according to the invention is defined by the features of claim 1. A device according to the invention is defined by the features of claim 16.

[0007] In the method of the invention, first the system time constant τ of the vacuum leak detector is determined. The system time constant τ represents the response time of the system with respect to a change in the state of the gas at the gas detector. In other words, the system time constant τ corresponds to the time of a system-related rise in the measurement signal due to the detection of the escape of a gas component from a leak in the test object and / or the time of a system-related fall in said measurement signal due to the disappearance of the detection of a gas component from the gas leak. In the simplest case, the system time constant τ corresponds, for example, to the elapsed time until the measurement signal decays to a fraction of 1 / e of the initial signal, where e is Euler's number.

[0008] After the system time constant is determined, the actual leak detection is performed by placing the device under test in the test chamber and evacuating the test chamber with a vacuum pump. A measurement signal I(t) of the gas drawn from the test chamber at a current time t is generated by the gas detector. The current time t can be, for example, the start of a new measurement or a new series of measurements. In the series of measurements, measurements can be taken at regular intervals t0.

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[0012] In the present invention, the presence or absence of a leak in the test object is determined based on the difference thus calculated, i.e., for example, I(t+t0)-C2×I(t+t0). For example, if this difference is very large, it may be determined that the gas detector detects an additional gas component that was not present before, and thus indicates a leak, whereas if the difference is very small, it may indicate that the conditions at the detector have not changed, i.e., the test object is dense. For this purpose, the difference may be compared, for example, with a threshold value. In this way, the use of knowledge and system time constants makes it possible to speed up leak testing and detection.

[0013] Preferably, the difference between the measurement signal prediction and the actual measured measurement signal at time t+t0 is multiplied by a constant C1 to numerically adjust the difference to match the actual amount of leakage. C1 and / or C2 are real numbers. Preferably, C2 is between 0 and 1 and C1 is greater than 1.

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[0015] For example, when measuring the partial pressure of a leaking gas (e.g., the leaking gas is helium), if the measurement signal rises due to the presence of the leaking gas, it can be determined that a leak is present when the difference exceeds a predetermined threshold, which can be 0. If the measurement signal falls due to the presence of the leaking gas, it can be determined that a leak is present in the test object when the difference falls below a predetermined threshold (e.g., 0).

[0016] Preferably, when comparing the difference with a threshold, it can be confirmed whether the difference a) exceeds the threshold, and / or b) is within a range from zero to the threshold, and / or c) is less than zero (i.e., negative). In the case of a), it can be determined that there is a leak in the test object. In the case of b), it can be determined that there is no leak in the test object. In the case of c), it can be determined that an error has occurred, for example, the system time constant is not accurate.

[0017] The threshold value may serve to take into account background signals, such as background noise of the measurement signal after a system time constant has elapsed. The background signal may be due to noise in the signal and / or an offset signal. The offset signal may be due to gas components released internally from the walls of the leak detection system. The threshold value may be set, for example, at 5 to 10 times the value of the background noise or background signal averaged over a given period of time.

[0018] It is further advantageous to use an average of the difference over a predetermined period when comparing the difference with the threshold value in order to ignore temporary outliers in the measurement signal.

[0019] The system time constant τ may be determined using a test leak (eg, a spray leak) or may be calculated from the volume of the test chamber and the suction capacity of the vacuum pump.

[0020] The system time constant τ is determined from the rate of rise or fall of the measurement signal (i.e., the measured leak volume signal) when the leak is connected or disconnected (i.e., when the leak is sprayed or when the leak spraying is stopped).

[0021] In this case, the system time constant τ can be directly determined from the time elapsed until the measured signal drops to 1 / e of the measurement signal generated or measured by, for example, a test leak. The system time constant τ can be directly determined from the time elapsed until the measured leak amount signal rises to (1-1 / e) times the measurement signal of the test leak.

[0022] Alternatively or additionally, the system time constant τ may be determined by the formula: τ=t / Ln(I(t=0))-In(I(t=t)), where I(t=0) is the measurement signal when the test leak is stopped, removed or turned off, and I(t=t) is the measurement signal at any time t after the test leak is stopped. In this case, the system time constant may be determined by taking I(t=0) in the above formula as the measurement signal obtained as the difference between the measurement signal before the test leak is started and the measurement signal at the time when the test leak is stopped, and taking I(t=t) as the difference between the measurement signal before the test leak is opened and the measurement signal at time t after the test leak is stopped.

[0023] The basic idea is to speed up the signal substantially. The system time constant of the system is a fixed value that can be measured technically by a leak detector by starting and stopping a suitable test leak. Knowing the time behavior, it is possible to predict how the signal will behave at any point in time when no test gas is currently being sprayed. This can be mathematically described by the following relationship:

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[0027] 1) Determine the system time constant for leak measurement using an appropriate method (built-in test leak, external test leak, spray leak, input of volume and suction capacity). 2) Optionally, filtering the leak signal to match the system time constant. 3) Speed ​​up the signal using the above formula. 4) Optionally, adjust the desired system time constant (boost factor) to match the test gas spray time, or adjust the spray time to match the system time constant setting. result: The signal is transformed as follows:

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[0029] (Where, I(t) is the leak rate signal at time t, I(t-τ / n) is the leak rate signal at time t-τ / n, F is the leak rate suppression coefficient (0.9...0.999), n is the boost coefficient (the magnitude of the virtual suction capacity compared to the actual suction capacity), and τ is the system time constant (which the system needs to measure in advance) that can be obtained, for example, from the effective suction capacity and volume, etc.)

[0030] As a result, the measurement signal speeds up. The system time constant of the system must be determined in advance, for example by using an internal test leak.

[0031] (Approach and basic philosophy) In a vacuum leak detector, the relationship between the effective suction capacity and the volume of the chamber determines the system time constant, i.e. the maximum speed at which the signal can rise or fall. In physical terms, a signal cannot rise or fall faster, it can only slow down due to, for example, outgassing or permeation of surfaces, evacuation of volumes not considered, etc. If we ignore all slowing effects, the signal behavior can always be described by a simple e-function:

[0032] For a rising signal, the function is:

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[0034] For a falling signal we have:

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[0036] Being an exponential function, the signal I(t) can be used to determine (the derivative) the maximum possible change in I(t) at any time. The maximum possible derivative (up or down) is always evident from the leak signal.

[0037] The basic idea is to treat the signal as if it were falling at any point in the signal, i.e., subtract the maximum physically possible change from the signal at any one time. Since this is an e function, it is appropriate to write it in units of the system time constant.

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[0041] Due to the exponential properties, the above formula can be greatly simplified:

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[0043] This makes it possible to examine how this new signal behaves under a variety of conditions.

[0044] (For rising signals)

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[0045] The result is thus, ideally, a time-independent signal with reduced amplitude. Time independence is always achieved if the signal at time t and the signal at time t-τ are both exactly on the rising edge of the signal. The signal before that is rising exponentially. What does this mean? When helium is sprayed, after a certain time (τ / n) the target amplitude is already reached. The signal is still rising, but the filtered signal already outputs the final value. To reach approximately the final value, two system time constants are usually required. A value of n=1 halves the effective system time constant, so to speak, i.e. the suction capacity is doubled. However, it is preferable to consider it as a virtual suction capacity. Values ​​significantly greater than 1 may also be used. In this way, the filter easily allows a virtual suction capacity of 200 l / s for the vacuum leak detector. As n increases, the measurement signal becomes even smaller:

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[0048] (For descending signals)

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[0049] Essentially, if you subtract the expected signal, you get a zero result, but only if the signal at time t and the signal at time t-τ / n are both constantly on a falling edge. If you spray helium, the signal will be visible almost exactly for time τ / n.

[0050] Of course, a value of zero is completely useless to a leak detector. Even a small error in the determination of the system time constant τ can result in a negative leak rate, which can lead to suction by the leak. Ideally, a small multiplication correction using the parameter F will result in a small but noticeable leak rate signal:

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[0052] F=0.9 reduces the displayed leak rate by one order of magnitude, and F=0.99 reduces the displayed leak rate by two orders of magnitude.

[0053] (For stable signals)

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[0055] (Summary of this filter) If you want to avoid negative leak values ​​and still want to see the exact leak amount, convert the signal to a filtered signal:

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[0057] (Where, I(t) is the leak rate signal at time t, I(t-τ / n) is the leak rate signal at time t-τ / n, F is the leak rate suppression coefficient (0.9...0.999), n is the boost coefficient (the magnitude of the virtual suction capacity compared to the actual suction capacity), and τ is the system time constant (which the system needs to measure in advance) that can be obtained, for example, from the effective suction capacity and volume, etc.)

[0058] (characteristic) The filter is essentially designed to speed up the signal of a blow-off leak (i.e., the effective suction capacity and all signals that vary with volume).

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[0060] (Prerequisite) The filter assumes that the system time constant is known with a sufficiently high accuracy. In an unknown system this can only be achieved by measurements. This is possible with a vacuum leak detector with an internal or external test leak. For this purpose, the test leak must be opened and at least wait until the signal is sufficiently stable. Of course, the test leak must be connected to the chamber. The system time constant can then be determined by switching off the test leak and measuring its decay time (e.g. the time to a fraction of 1 / e of the initial signal). Of course, for large volumes and small effective suction capacities this may take up to a minute. On the other hand, it also gives the user information about the system time constant, i.e. the required leak spray time. Due to the extremely short measurement times, the time spent determining the system time constant is quickly compensated for by a few measurement points.

[0061] (noise) The filtered signal is much noisier than the original leak detector signal, because at least two signal values ​​are needed each time and the useful signal becomes even smaller as n increases. However, the signal is small for large volumes and short spray times, so the leak detector must give a very small indication of the leak volume. However, the filter compensates for this. Therefore, even a large background noise does not have a significant disturbing effect.

[0062] Hereinafter, embodiments will be described with reference to the drawings. [Brief description of the drawings]

[0063] [Figure 1] FIG. 2 is a diagram of the signal path obtained in the first embodiment. [Diagram 2] FIG. 1 is a schematic diagram of a vacuum leak detector. [Diagram 3]FIG. 13 is a diagram showing how a system time constant of a system is determined. [Figure 4] FIG. 11 is another diagram showing how a system time constant is determined. [Diagram 5] FIG. 11 is yet another diagram showing how a system time constant is determined. [Figure 6] FIG. 13 shows an example of filtering and illustrates a speed-up signal and a system time constant. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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[0065] 2 is a schematic diagram of a vacuum leak detector comprising a test chamber 12, a gas detector 14 and a vacuum pump 16. The gas detector 14 is gas-conductively connected to the test chamber 12 by a gas conduit 18. The test chamber contains a test object 20 filled with a test gas. The gas detector 14 is electronically connected to an evaluation unit 22 which receives and processes electronic measurement signals generated by the gas detector.

[0066] In another embodiment, a UL1000 with a 50 liter barrel was used. A Limpventil was used with a TI4-6 as a controllable test leak. The data input to this filter was combined with a fixed filter to avoid effects of different filtering times and to allow for a measurement of noise amplification.

[0067] Various tests were performed by opening and closing the limp valve to examine the signal at various speed stages. Specifically, we investigated the signal drop, signal rise, and the question of whether the filter is consistent in predicting the amount of leakage or the degree of difference. The scope of the investigation of this filter is 1×10 -3 ~1×10 -9 mbar×l / sec.

[0068] 1) This filter works well. 2) The signal can be effectively sped up by a factor of 32, allowing a virtual suction capacity of nearly 1000 l / sec for a typical leak detector. 3) A built-in test leak allows the system time constant required for the filter to be determined, which takes 15 to 20 seconds. 4) Compared to the input signal, the accelerated signal has a factor (1.4) increase in noise, which is consistent with the theoretical assumptions. 5) However, for short duration spray pulses (short relative to the system time constant), the noise increases by a factor of (1.4), assuming the speed-up factor is not too high. The noise is proportional to the spray time. 8) The leakage prediction is good, and this filter has very little overload or underload of the signal. 9) By smoothing the input signal to match the actual system time constant, noise is significantly improved without significantly deteriorating the system time constant. 10) For a volume of 50 liters, the waiting time before spraying after a large leak can be reduced to approximately 1 minute.

[0069] FIG. 3 is a diagram showing an example of determining the system time constant of a leak detection system, in which the rise or fall of a logarithmic measurement signal is measured.

[0070] FIG. 4 shows an example of direct determination of the system time constant, where the time it takes for the measurement signal to drop to a rate of 1 / e is the measurement of interest.

[0071] FIG. 5 shows an example of directly determining the system time constant, where the time elapsed until the measured signal rises to a rate of (1-1 / e) is the measurement of interest.

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Claims

【Request 1】 【Number 1】

2. 2. The method of claim 1, further comprising determining that a leak has been detected in the device under test if the difference exceeds a threshold value.

3. 3. The method according to claim 1, further comprising determining that the object under test is dense if the difference is greater than or equal to 0 and less than or equal to a threshold value.

4. 4. The method according to claim 1, characterized in that if the difference is less than 0, it is determined that there is an error, e.g. an inaccurate system time constant.

5. 5. The method according to claim 2, wherein the threshold value is r times the value of the background signal of the measurement signal, r being a rational number greater than 0, preferably greater than or equal to 5 and / or less than or equal to 10. 【Request 6】 【Number 2】 【Request 7】 【Number 3】 【Request 8】 【Number 4】

9. 9. The method according to any one of the preceding claims, characterized in that the second constant C2 comprises the n-th root of Euler's number e, preferably the reciprocal of the n-th root of e.

10. 10. The method according to any one of claims 1 to 9, characterized in that the second constant C2 is multiplied by a factor F, F being a rational number less than 1, preferably between 0.9 and 0.

999. 【Request 11】 【Number 5】

12. 12. The method according to any one of claims 1 to 11, characterized in that the system time constant τ is determined using a test leak, which can be a built-in test leak of the vacuum leak detector or an external test leak that can be connected to the test chamber (12).

13. 13. The method according to claim 1, wherein the vacuum time constant of the vacuum leak detector is calculated as the system time constant τ from the volume of the test chamber (12) and the piping lines connecting the test chamber (12) to the gas detector (14) and the suction capacity of the vacuum pump (16).

14. 14. The method according to any one of claims 1 to 13, characterized in that the system time constant τ is determined from the time that elapses between the time that a test leak is switched off, stopped or removed from the vacuum leak detector and the time that the measurement signal decays to a predetermined value, or from the time that elapses between the time that a test leak is switched on, activated or added to the vacuum leak detector and the time that the measurement signal rises to a predetermined value.

15. 15. The method according to claim 1, wherein the predetermined time corresponds to a time when the measurement signal is 1 / e times the steady state measurement signal of the test leak.

16. A vacuum leak detector for carrying out the method according to any one of claims 1 to 15, comprising: A vacuum pump (16); a test chamber (12) connected to the vacuum pump (16); a gas detector (14) connected to the test chamber (12); The vacuum leak detector further comprises: An evaluation unit (22) for evaluating a measurement signal of the gas detector (14), the evaluation unit (22) being configured to perform the method according to any one of claims 1 to 15, A vacuum leak detector comprising:

17. 17. A vacuum leak detector according to claim 16, characterized in that the evaluation unit (22) includes a memory in which a processing procedure is stored.

18. 18. A vacuum leak detector according to any one of the preceding claims, characterized in that the evaluation part (22) comprises a microcontroller arranged to perform the method automatically.

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